Loop Heat pipes (LHPs) and Capillary Pumped Loops (CPLs) are passive two-phase heat transport systems that utilize the capillary pressure developed in a fine pored evaporator wick to circulate the system's working fluid. CPLs, which were developed in the United States, typically feature one or more capillary pumps or evaporators, while LHPs, which originated in the former Soviet Union, are predominantly single evaporator systems. The primary distinguishing characteristic between the two systems is the location of the loop's reservoir, which is used to store excess fluid displaced from the loop during operation. A reservoir of a CPL is located remotely from the evaporator and is cold biased using either the sink or the subcooled condensate return. On the other hand, the reservoir of an LHP is thermally and hydraulically coupled to the evaporator. This difference in reservoir location is responsible for the primary difference in the behavior of the two devices.
Referring to FIG. 1, the separation of the reservoir 110 from the plural, parallel evaporators 120 in a CPL is schematically illustrated. This separation makes it possible to construct thermal management loops that can incorporate any combination of series connected or parallel connected evaporators 120 and/or condensers 130.
This feature offers distinct advantages for applications that require heat dissipation from large payload footprints or multiple separated heat sources. CPL's have also demonstrated highly desirable thermal control/management properties such as sensitive temperature control properties that require only very modest application of heat to its reservoir, highly effective heat load sharing between evaporators that can totally eliminate the need for any heater energy to maintain inactive equipment at safe-mode temperatures, and heat sink (condenser) diode action which can provide protection from temporary exposure to hot environments.
Unfortunately, the advantages derived from a separated (remotely located) reservoir result in significant disadvantages that have limited the further evolution and application of CPL's. For example, CPL's are disadvantaged during start-up because the loop must first be preconditioned by heating the reservoir to prime the evaporator's wick before the heat source can be cooled. The principle disadvantage of CPL's, however, is its total reliance on subcooled liquid return to maintain stable operation at each and every evaporator capillary pump. As a consequence, CPL's require low conductivity wick materials to minimize their reliance on subcooling and impose constraints on tolerable system power and/or environment temperature cycling conditions.
On the other hand, referring to FIG. 2, a reservoir 210 of a LHP is co-located with the evaporator 220 and is thermally and hydraulically coupled to it with a conduit 230 that contains a capillary link 234 often referred to as a secondary wick. The interconnecting conduit 230 makes it possible to vent any vapor and/or bubbles of non-condensible gas (or “NCG bubbles”) from the core of the evaporator 220 to the reservoir 210. The capillary link 234, on the other hand, makes it possible to pump liquid from the reservoir 210 to the evaporator 220. This insures a wetted primary wick 224 during start-up, and prevents liquid depletion of the primary wick 224 during normal steady state operation and during transient temperature conditions of either the heat source 240 or the heat sink 250 (adjacent the condenser 260). This architecture makes LHP's extremely robust and reliable, and makes preconditioning during start-up unnecessary. The control of vapor and liquid in the pump core provided by the secondary wick 234 minimizes the reliance of the loop on liquid subcooling. As a result, LHP's utilize metallic wicks, which offer an order of magnitude improvement in pumping capacity over the low conductivity wicks that are typically used in CPL's.
The problem with “robust” LHP's is that they are limited to single evaporator/reservoir designs, which limit their application to heat sources with relatively small thermal footprints.
Ideally, a true thermal bus should incorporate the unrestricted combination of multiple evaporators and thermal management properties of a CPL together with the reliability and robustness of an LHP. One impediment to even greater utilization of the LHP is its limitation to single evaporator systems. Many applications require thermal control of large payload footprints or multiple separated heat sources that are best served by multiple evaporator LHP's, which ideally would offer the same reliability and robustness as their single evaporator predecessors.
Several investigators have previously experimented with multiple evaporator LHP's with mixed results. The effort of these investigators, summarized below, indicates that multiple evaporator LHP's are only marginally feasible. These multiple evaporator LHP's are limited in the number of evaporators that can be plumbed in parallel and/or are limited in the spatial separation between the evaporators.
Bienert et al. developed a breadboard LHP with two evaporators, each with its own compensation chamber (reservoir). Although the loop, which was charged with water, was designed without rigorous sizing and seemed to be sensitive to non-condensible gas, the breadboard made a proof-of-principle demonstration of the feasibility of a dual evaporator LHP. For further details, refer to Bienert, W., Wolf, D., and Nikitkin, M., “The Proof-Of-Feasibility Of Multiple Evaporator Loop Heat Pipe”, 6th European Symposium on Environmental Systems, May 1997.
More recently, the inventors of the present invention developed and demonstrated reliable operation of a dual evaporator LHP system, with a separate reservoir to each evaporator pump, was using ammonia as working fluid. Referring to FIG. 3, a schematic view of this dual evaporator LHP is illustrated. It has two parallel evaporator pumps 310, 320, each with its own reservoir 312, 322, vapor transport lines 314, 324, and liquid transport lines 316, 326, and a direct condensation condenser 330. The reservoirs 312, 322 were sized and the system charged to allow one reservoir to completely fill with liquid while the other reservoir remained partially filled at all operating conditions. The dual evaporator/dual reservoir design clearly demonstrated comparable reliability and robustness as its single evaporator predecessors. For further details, refer to Yun, S., Wolf, D., and Kroliczek, E., “Design and Test Results of Multi-Evaporator Loop Heat Pipe”, SAE Paper No. 1999-01-2051, 29th International Conference on Environmental Systems, July 1999.
However, there is limitation on the number of evaporators that can be reasonably used in multiple reservoir systems that are designed to operate over a wide temperature range. Referring to FIG. 4, a graphical analysis of hydro-accumulator sizing is illustrated for a typical LHP system designed for a maximum operating temperature of 65° C. As the minimum operating temperature decreases, and the hydro-accumulator volume increases rapidly as the number of evaporators increases. As an example, at a minimum operating temperature of −40° C., the volume of each hydro-accumulator increases by a factor of three between a two-evaporator system and a three-evaporator system. Over the same operating temperature range, a four-evaporator system would require an infinite hydro-accumulator volume.
Van Oost et al. developed a High Performance Capillary Pumping Loop (HPCPL) that included three parallel evaporators connected to the same reservoir. Referring to FIG. 5, a schematic view of the basic design of the HPCPL loop is illustrated. The reservoir 510 was co-located at the evaporator end of the loop, and included capillary links 512, 514 between the evaporators 522, 524 and the reservoir 510, making the device similar to a LHP. The loop has been successfully tested on the ground with a favorable gravitational bias of the evaporators relative to the reservoir. This orientation constraint is due to limits imposed by the capillary links 512, 514. For further details, refer to Van Oost et al., “Test Results of Reliable and Very High Capillary Multi-Evaporator/Condenser Loop”, 25th International Conference on Environmental Systems, Jul. 10-13, 1995.
Although this concept represents some advantages over a single evaporator LHP design, the capillary link 512, 514 connecting the evaporators 522, 524 to the reservoir 510 limits the separation between the evaporators and the reservoir. This limitation is similar to the transport and orientation limitations normally encountered with conventional heat pipes, as described by Kotlyarov et al., “Methods of Increase of the Evaporators Reliability for Loop Heat Pipes and Capillary Pumped Loop”, 24th International Conference on Environmental Systems, Jun. 20-23, 1994.
The robustness of an LHP is derived from its ability to purge vapor/NCG bubbles via a path 516, 518 from the liquid core of the evaporator 522, 524 to the reservoir 510. The disadvantage of the LHP is the limitation imposed by the heat pipe like characteristics of the capillary link. Hoang suggested (in a document entitled “Advanced Capillary Pumped Loop (A-CPL) Project Summary”, Contract No. NAS5-98103, March 1994) that such a link could itself be a loop and incorporated the idea in an Advanced Capillary Pumped Loop (A-CPL) concept which incorporates both the advantages of a robust LHP and the architectural flexibility of a CPL. An A-CPL system has been successfully co-developed and demonstrated by TTH Research, Inc. and Swales Aerospace.
Referring to FIG. 6, a schematic view of the A-CPL concept is illustrated. The ACPL contains two conjoint independently operated loops—a main loop and an auxiliary loop. The main loop is basically a traditional CPL whose function is to transport the waste heat QV input at the evaporator capillary pump 610 and reject it to a heat sink via the primary condenser 620. Hence, hardware and operational principles of the main loop are similar to those of a CPL. The auxiliary loop is utilized to remove vapor/NCG bubbles from the core of the evaporator capillary pump 610 and the reservoir capillary pump 630 and move them to the two-phase reservoir 640. The auxiliary loop also provides QR heat transport from the reservoir capillary pump 630 to heat sinks via the auxiliary condenser 650 and the primary condenser 620. In addition, the auxiliary loop is also employed to facilitate the start-up process. In this manner, the auxiliary loop functionally replaces the secondary wick in a conventional LHP.
An A-CPL prototype was fabricated and tested with the goal of demonstrating the basic feasibility of the concept. Referring to FIG. 7, a schematic view of the prototype loop is illustrated. The A-CPL prototype consisted of two 3-port nickel CPL evaporator pumps 710, 720 with a secondary loop driven by a reservoir capillary pump 730. For this prototype, the reservoir capillary pump 730 was a “short” evaporator loop heat pipe (LHP), whose hydro-accumulator 732 also serves as the entire system's reservoir. The LHP was used as the reservoir capillary pump 730 only to verify the functionality of the secondary loop. In its final form, the A-CPL would be equipped with an reservoir capillary pump that is optimized for its specific function. Testing demonstrated the feasibility of:                Operation of multiple, small diameter (<1″ OD) metal nickel wick        Startup without pressure priming and liquid clearing of vapor line. (typical CPL startup process)        Quick startup        Robust operation under severe operational conditions (low power, power cycling, condenser cycling)        
However, the above demonstration was achieved in series connected evaporator configuration only. This means that the secondary flow created by the reservoir capillary pump 730 flowed through the liquid cores of the evaporator pumps 710, 720 in series. Several tests were also conducted in parallel configuration. Results showed that the secondary flow preferentially went to the #1 evaporator pump 710, which has slightly less impedance in its liquid inlet line section than the #2 evaporator pump 720. This bias toward the #1 evaporator pump 710 made testing in a parallel configuration difficult to characterize.